1. Field of the Invention
The present invention relates to a waveguide slot antenna, and particularly to a structure of a waveguide slot antenna capable of generating a circularly polarized wave.
2. Description of the Related Art
A waveguide slot antenna utilizing a waveguide as a feeding line has been used as an antenna element usable in a microwave band and a millimeter-wave band, in a base station for wireless communication terminals.
In use for wireless communication terminals, there are some situations in which, rather than a linearly polarized wave, a circularly polarized wave less susceptible to fading is desirable. Therefore, in connection with the need for a waveguide slot antenna adapted to radiate a circularly polarized wave, circularly polarized wave antenna devices using various waveguide slot antennas have been proposed.
Basically, a waveguide slot antenna having a linear-shape slot radiates a linearly polarized wave therefrom.
Thus, in a conventional waveguide slot antenna for radiating a circularly polarized wave, a linearly polarized wave is converted into a circularly polarized wave by combining a pair of linear slots to generate mutually orthogonal polarized waves, as disclosed in the following Non-Patent Documents 1 to 5. JP 2012-065229A discloses a waveguide slot antenna in which a linearly polarized wave is converted into a circularly polarized wave by coupling a parasitic element to a linear slot to generate orthogonal polarized wave components.
FIG. 11 is a perspective view illustrating an example of a conventional waveguide slot antenna for radiating a circularly polarized wave. As illustrated in FIG. 11, one wall section of a hollow waveguide 89 has a cross slot 59 composed of a combination of two linear slots 79, 79 and provided offset from a center line CL thereof parallel to an axis of the hollow waveguide 89.